Unveiling the enhancement mechanism of NH3-SCR performance in Mn-modified α-Fe2O3 on Fe2O3-TiO2 catalysts with varied crystal facets

This study explored the impact of Mn modification and diverse crystal facets on iron-based catalysts, focusing on Mn-modified Fe₂O₃-TiO₂ catalysts with various α-Fe₂O₃ crystal facets ({012}, {014}, and {113}). The modification of α-Fe₂O₃ with manganese significantly enhanced the NH₃-SCR performance of the Mn/Fe₂O₃-TiO₂ catalysts exhibiting varying effects based on the different exposed crystal facets. Notably, the Mn/Fe₂O₃{113}-TiO₂ catalyst demonstrated the highest efficacy, maintaining NO_x conversion rates exceeding 90.0 % in the temperature range of 200–350 °C, peaking near 100.0 % between 250 and 300 °C, and N₂ selectivity above 70.0 % within 150–300 °C. The {113} crystal facet of α-Fe₂O₃, characterized by its higher surface energy, enhanced the presence of surface defect species, thereby promoting the adsorption and activation of reactants while facilitating interactions among Fe, Mn, and Ti. These enhanced interactions facilitated electron migration, increasing the release of Mn⁴⁺ and O_α species, which in turn increased the redox capacity. This intrinsic mechanism contributed to the superior activity of the Mn/Fe₂O₃{113}-TiO₂ catalyst. Both the Mn/Fe₂O₃{014}-TiO₂ and Mn/Fe₂O₃{113}-TiO₂ catalysts exhibited NH₃ adsorption on both Brønsted acid sites and Lewis acid sites. The adsorbed NH₃ species reacted with gaseous NO species and adsorbed nitrate species through Langmuir-Hinshelwood (L-H) reaction and Eley-Rideal (E-R) mechanisms. Additionally, Mn⁴⁺ promoted a "Fast SCR" reaction, significantly boosting the catalysts’ activity at low temperatures. The Mn/Fe₂O₃{113}-TiO₂ catalyst, with elevated Mn⁴⁺ and O_α levels, exhibited increased efficiency in both "Standard SCR" and "Fast SCR" pathways. This study provides valuable insights for developing high-performance α-Fe₂O₃-based NH₃-SCR catalysts with extended operating temperature ranges.